In the beginning, William wasn't planning on doing anything. However, when Prince Aizel was about to grab Brianna's hand, his body moved subconsciously.
When he came to himself, he was already grabbing the Second Prince's hand and looking at the latter's glaring face.
Perhaps, due to himself being a victim of an arranged marriage, William felt that Brianna was very pitiful. She had just turned eleven, but was now forced to marry someone she didn't even like.
For a brief moment, the image of Brianna's clear and confident eyes appeared inside William's mind. In the next instant, those same eyes clouded over and lost their brilliance. Deep inside his heart, the Half-Elf didn't want the lively young lady to lose her radiance.